U.S. patent application number 10/640176 was filed with the patent office on 2004-04-29 for multimode ionization source.
Invention is credited to Bertsch, James L., Fischer, Steven M., Gourley, Darrell L..
Application Number | 20040079881 10/640176 |
Document ID | / |
Family ID | 33565255 |
Filed Date | 2004-04-29 |
United States Patent
Application |
20040079881 |
Kind Code |
A1 |
Fischer, Steven M. ; et
al. |
April 29, 2004 |
Multimode ionization source
Abstract
The present invention provides an apparatus and method for use
with a mass spectrometer. The multimode ionization source of the
present invention provides one or more atmospheric pressure
ionization sources (e.g., electrospray, atmospheric pressure
chemical ionization and/or atmospheric pressure photoionization)
for ionizing molecules. A method of producing ions using the
multimode ionization source is also disclosed. The apparatus and
method provide the advantages of the combined ion sources without
the inherent disadvantages of the individual sources. In an
embodiment, the multimode ionization source includes an infrared
emitter enclosed in an inner chamber for drying a charged aerosol.
ESI/APCI multimode sources may include a corona needle shield
and/or an auxiliary electrode.
Inventors: |
Fischer, Steven M.;
(Hayward, CA) ; Gourley, Darrell L.; (San
Francisco, CA) ; Bertsch, James L.; (Palo Alto,
CA) |
Correspondence
Address: |
AGILENT TECHNOLOGIES, INC.
Legal Department, DL429
Intellectual Property Administration
P. O. Box 7599
Loveland
CO
80537-0599
US
|
Family ID: |
33565255 |
Appl. No.: |
10/640176 |
Filed: |
August 13, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10640176 |
Aug 13, 2003 |
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10245987 |
Sep 18, 2002 |
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6646257 |
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Current U.S.
Class: |
250/288 ;
250/423P; 250/423R; 250/424 |
Current CPC
Class: |
H01J 49/162 20130101;
H01J 49/107 20130101; H01J 49/168 20130101; H01J 49/0445 20130101;
H01J 49/165 20130101 |
Class at
Publication: |
250/288 ;
250/423.00R; 250/423.00P; 250/424 |
International
Class: |
H01J 049/10; H01J
027/02 |
Claims
What is claimed is:
1. A multimode ionization source, comprising: (a) an electrospray
ionization source for providing a charged aerosol; (b) an infrared
emitter adjacent to the electrospray ionization source for drying
the charged aerosol; (c) an atmospheric pressure ionization source
downstream from the electrospray ionization source for further
ionizing said charged aerosol; and (d) a conduit adjacent to the
atmospheric pressure ionization source and having an orifice for
receiving ions from the charged aerosol.
2. The multimode ionization source of claim 1, wherein the
atmospheric pressure ionization source is an atmospheric pressure
photo-ionization (APPI) source.
3. The multimode ionization source of claim 1, wherein the
atmospheric pressure ionization source is an atmospheric pressure
chemical ionization (APCI) source.
4. The multimode ionization source of claim 1, further comprising:
a first electrode interposed between the electrospray ionization
source and the conduit; and a second electrode interposed between
the first electrode and the orifice for guiding ions toward the
orifice.
5. The multimode ionization source of claim 1, wherein the infrared
emitter comprises an infrared (IR) lamp situated within an
enclosure.
6. The multimode ionization source of claim 5, wherein the
enclosure is configured to confine heat arising from the infrared
lamp within the enclosure, and the enclosure includes an exit
adjacent to the orifice of the conduit.
7. The multimode ionization source of claim 1, wherein the infrared
emitter radiates at a wavelength between about 2 and 6 microns.
8. The multimode ionization source of claim 1, wherein the
electrospray ionization source has a longitudinal axis and the
conduit has a longitudinal axis and wherein the longitudinal axis
of the electrospray ionization source is substantially orthogonal
to the longitudinal axis of the conduit.
9. A mass spectrometer for multimode ion production, comprising:
(a) a multimode ionization source comprising: i. an electrospray
ionization source for providing a charged aerosol; ii. an infrared
emitter adjacent to the electrospray ionization source for drying
the charged aerosol; iii. an atmospheric pressure ionization source
downstream from the electrospray ionization source for further
ionizing said charged aerosol; and iv. a conduit adjacent to the
atmospheric pressure ionization source and having an orifice for
receiving ions from the charged aerosol; and (b) a detector
downstream from the multimode ionization source for detecting the
ions produced by the multimode ionization source.
10. The mass spectrometer for multimode ion production of claim 9,
wherein the atmospheric pressure ionization source is an
atmospheric pressure photo-ionization (APPI) source.
11. The mass spectrometer for multimode ion production of claim 9,
wherein the atmospheric pressure ionization source is an
atmospheric pressure chemical ionization (APCI) source.
12. The mass spectrometer for multimode ion production of claim 9,
wherein the multimode ionization source further comprises: a first
electrode interposed between the electrospray ionization source and
the conduit; and a second electrode interposed between the first
electrode and the orifice for guiding ions toward the orifice of
the conduit.
13. The mass spectrometer for multimode ion production of claim 9,
wherein the infrared emitter comprises an infrared (IR) lamp
situated within an enclosure.
14. The mass spectrometer for multimode ion production of claim 9,
wherein the electrospray ionization source has a longitudinal axis
and the conduit has a longitudinal axis and wherein the
longitudinal axis of the electrospray ionization source is
substantially orthogonal to the longitudinal axis of the
conduit.
15. A method of producing ions using a multimode ionization source,
comprising: (a) producing a charged aerosol by electrospray
ionization; (b) exposing the charged aerosol to infrared radiation,
the infrared radiation drying the aerosol; (c) further ionizing the
charged aerosol using an atmospheric pressure ionization source;
and (d) detecting the ions produced from the multimode ionization
source.
16. The method of claim 15, wherein the atmospheric pressure
ionization source is an atmospheric pressure photo-ionization
(APPI) source.
17. The method of claim 15, wherein the atmospheric pressure
ionization source is an atmospheric pressure chemical ionization
(APCI) source.
18. The method of claim 15, further comprising: (e) guiding the
charged aerosol downstream using electrodes.
19. The method of claim 15, further comprising: (f) confining the
charged aerosol within an enclosed area as it is exposed to the
infrared radiation.
20. A multimode ionization source, comprising: (a) a source
housing; (b) a nebulizer disposed in the source housing and having
an orifice for providing a charged aerosol; (c) a corona needle
disposed in the housing and positioned downstream from the
nebulizer for further ionizing the charged aerosol; (d) a shield
substantially enclosing the corona needle; and (e) a conduit having
an orifice adjacent to the corona needle for receiving ions from
the charged aerosol.
21. The multimode ionization source of claim 20, further
comprising: (f) a drying device adjacent to the orifice of the
nebulizer for drying the charged aerosol;
22. The multimode ionization source of claim 20, wherein the shield
is configured to substantially isolate the corona needle from a
flow of the charged aerosol.
23. The multimode ionization source of claim 22, wherein the shield
substantially surrounds the corona needle and has an exit allowing
passage of a discharge from the corona needle.
24. The multimode ionization source of claim 21, wherein the drying
device comprises an infrared emitter.
25. The multimode ionization source of claim 20, wherein a voltage
is applied to the shield.
26. The multimode ionization source of claim 20, further
comprising: (g) a first electrode interposed between the orifice of
the nebulizer and the orifice of the conduit for producing ions
from the orifice of the nebulizer; and (h) a second electrode
interposed between the first electrode and the orifice of the
conduit and positioned upstream from the corona needle for
directing ions from the first electrode toward the orifice of the
conduit.
27. A method of producing ions using a multimode source including
an ESI source and an APCI source, comprising: (a) producing a
charged aerosol using the ESI source; (b) producing a discharge
with a corona needle having a shield; and (c) exposing the charged
aerosol to the discharge.
28. The method of claim 27, further comprising: (d) drying the
charged aerosol produced by the ESI source.
29. The method of claim 28, wherein the drying comprises exposing
the charged aerosol to an emission of infrared radiation.
30. The method of claim 27, wherein the shield substantially
surrounds the corona needle and has an exit for allowing passage of
the discharge.
31. The method of claim 27, further comprising: (d) guiding the
charged aerosol after exposure to the discharge toward an entrance
of a mass analyzer by subjecting the charged aerosol to an electric
field.
32. The method of claim 31, wherein the electric field is generated
by applying a voltage to the shield.
33. A multimode ionization source, comprising: (a) a source
housing; (b) a nebulizer disposed in the source housing and having
an orifice for providing a charged aerosol; (c) a corona needle
disposed in the housing and positioned downstream from the
nebulizer for further ionizing the charged aerosol; (d) an
auxiliary electrode adjacent to the corona needle; and (e) a
conduit having an orifice adjacent to the corona needle for
receiving ions from the charged aerosol.
34. The multimode ionization source of claim 33, further
comprising: (f) a drying device adjacent to the orifice of the
nebulizer for drying the charged aerosol.
35. The multimode ionization source of claim 34, wherein the drying
device comprises an infrared emitter.
36. The multimode ionization source of claim 33, wherein the
auxiliary electrode is positioned between the corona needle and the
orifice of the conduit and has a portion extending toward the
corona needle, the auxiliary electrode generating an electric field
that guides ions in the charged aerosol toward the orifice.
37. The multimode ionization source of claim 33, further
comprising: (g) a second electrode interposed between the orifice
of the nebulizer and the orifice of the conduit for producing ions
from the orifice of the nebulizer; and (h) a third electrode
interposed between the first electrode and the orifice of nebulizer
and positioned upstream from the corona needle for directing ions
from the first electrode toward the orifice of the conduit.
38. A method of producing ions using a multimode source including
an ESI source and an APCI source having a corona needle, the
comprising: (a) producing a charged aerosol using the ESI source;
(b) exposing the charged aerosol to a discharge from the corona
needle, further ionizing the charged aerosol; and (c) guiding ions
in the charged aerosol using a auxiliary electrode after the
charged aerosol is exposed to the discharge.
39. The method of claim 38, further comprising: (d) drying the
charged aerosol produced by the ESI source.
40. The method of claim 39, wherein the drying comprises exposing
the charged aerosol to an emission of infrared radiation.
41. The method of claim 38, wherein the auxiliary electrode has a
portion extending toward the corona needle.
42. The method of claim 38, wherein the auxiliary electrode guides
ions in the charged aerosol toward an entrance of a mass analyzer.
Description
RELATED APPLICATIONS
[0001] The present application is a continuation-in-part of pending
U.S. patent application Ser. No. 10/245,987, filed Sep. 18,
2002.
FIELD OF THE INVENTION
[0002] The invention relates generally to the field of mass
spectrometry and more particularly toward an atmospheric pressure
ion source (API) that incorporates multiple ion formation
techniques into a single source.
BACKGROUND INFORMATION
[0003] Mass spectrometers work by ionizing molecules and then
sorting and identifying the molecules based on their mass-to-charge
(m/z) ratios. Two key components in this process include the ion
source, which generates ions, and the mass analyzer, which sorts
the ions. Several different types of ion sources are available for
mass spectrometers. Each ion source has particular advantages and
is suitable for use with different classes of compounds. Different
types of mass analyzers are also used. Each has advantages and
disadvantages depending upon the type of information needed.
[0004] Much of the advancement in liquid chromatography/mass
spectrometry (LC/MS) over the last ten years has been in the
development of new ion sources and techniques that ionize analyte
molecules and separate the resulting ions from the mobile phase.
Earlier LC/MS systems performed at sub-atmospheric pressures or
under partial vacuum, whereas API occurs at atmospheric pressure.
In addition, historically in these older systems all components
were generally under vacuum, whereas API occurs external to the
vacuum and the ions are then transported into the vacuum.
[0005] Previous approaches were successful only for a very limited
number of compounds.
[0006] The introduction of API techniques greatly expanded the
number of compounds that can be successfully analyzed using LC/MS.
In this technique, analyte molecules are first ionized at
atmospheric pressure. The analyte ions are then spatially and
electrostatically separated from neutral molecules. Common API
techniques include: electrospray ionization (ESI), atmospheric
pressure chemical ionization (APCI) and atmospheric pressure
photoionization (APPI). Each of these techniques has particular
advantages and disadvantages.
[0007] Electrospray ionization is the oldest technique and relies
in part on chemistry to generate analyte ions in solution before
the analyte reaches the mass spectrometer. The LC eluent is sprayed
(nebulized) into a chamber at atmospheric pressure in the presence
of a strong electrostatic field and heated drying gas. The
electrostatic field charges the LC eluent and the analyte
molecules. The heated drying gas causes the solvent in the droplets
to evaporate. As the droplets shrink, the charge concentration in
the droplets increases. Eventually, the repulsive force between
ions with like charges exceeds the cohesive forces and the ions are
ejected (desorbed) into the gas phase. The ions are attracted to
and pass through a capillary or sampling orifice into the mass
analyzer. Some gas-phase reactions, mostly proton transfer and
charge exchange, can also occur between the time ions are ejected
from the droplets and the time they reach the mass analyzer.
[0008] Electrospray is particularly useful for analyzing large
biomolecules such as proteins, oligonucleotides, peptides etc. The
technique can also be useful for analyzing polar smaller molecules
such as benzodiazepines and sulfated conjugates. Other compounds
that can be effectively analyzed include ionizing salts and organic
dyes.
[0009] Large molecules often acquire more than one charge. Multiple
charging provides the advantage of allowing analysis of molecules
as large as 150,000 u even though the mass range (or more
accurately mass-to-charge range) for a typical LC/MS instrument is
around 3000 m/z. When a large molecule acquires many charges, a
mathematical process called deconvolution may be used to determine
the actual molecular weight of the analyte.
[0010] A second common technique performed at atmospheric pressure
is atmospheric pressure chemical ionization (APCI). In APCI, the LC
eluent is sprayed through a heated vaporizer (typically
250-400.degree. C.) at atmospheric pressure. The heat vaporizes the
liquid and the resulting gas phase solvent molecules are ionized by
electrons created in a corona discharge. The solvent ions then
transfer the charge to the analyte molecules through chemical
reactions (chemical ionization). The analyte ions pass through a
capillary or sampling orifice into the mass analyzer. APCI has a
number of important advantages. The technique is applicable to a
wide range of polar and nonpolar molecules. The technique rarely
results in multiple charging like electrospray and is, therefore,
particularly effective for use with molecules of less than 1500 u.
For these reasons and the requirement of high temperatures, APCI is
a less useful technique than electrospray in regards to large
biomolecules that may be thermally unstable. APCI is used with
normal-phase chromatography more often than electrospray is because
the analytes are usually nonpolar.
[0011] Atmospheric pressure photoionization for LC/MS is a
relatively new technique. As in APCI, a vaporizer converts the LC
eluent to the gas phase. A discharge lamp generates photons in a
narrow range of ionization energies. The range of energies is
carefully chosen to ionize as many analyte molecules as possible
while minimizing the ionization of solvent molecules. The resulting
ions pass through a capillary or sampling orifice into the mass
analyzer. APPI is applicable to many of the same compounds that are
typically analyzed by APCI. It shows particular promise in two
applications, highly nonpolar compounds and low flow rates (<100
ul/min), where APCI sensitivity is sometimes reduced. In all cases,
the nature of the analyte(s) and the separation conditions have a
strong influence on which ionization technique: electrospray, APCI,
or APPI will generate the best results. The most effective
technique is not always easy to predict.
[0012] Each of these techniques described above ionizes molecules
through a different mechanism. Unfortunately, none of these
techniques are universal sample ion generators. While many times
the lack of universal ionization could be seen as a potential
advantage, it presents a serious disadvantage to the analyst
responsible for rapid analysis of samples that are widely
divergent. An analyst faced with very limited time and a broad
array of numerous samples to analyze is interested in an ion source
capable of ionizing as many kinds of samples as possible with a
single technique and set of conditions. Unfortunately, such an API
ion source technique has not been available.
[0013] Attempts have been made to improve sample ionization
coverage by the use of rapid switching between positive and
negative ion detection. Rapid positive/negative polarity switching
does result in an increase in the percentage of compounds detected
by any API technique. However, it does not eliminate the need for
more universal API ion generation.
[0014] For these reasons it would be desirable to employ a source
that can provide the benefits of multiple sources (electrospray,
APCI, and APPI) combined, but not have the individual limitations.
In addition, it would be desirable to have a source which does not
require switching from one source to another source or which
requires manual operations to engage the source. Thus, there is a
need to provide a multimode ion source that can ionize a variety of
samples quickly, efficiently and effectively.
[0015] To best accommodate two or more different ionization sources
in a single ion source apparatus, it is advantageous to avoid
having one ionization source mechanism interfere with the other
ionization source mechanism(s). One concern that may arise when an
ESI source is used in conjunction with another ionization source is
ensuring effective drying of the aerosol containing the analyte
ions. Since ESI sources normally do not use a vaporizer tube
because of the possibility of ion discharge to walls of the tube,
it is particularly advantageous to provide an alternative technique
for drying the aerosol that does not interfere with either the
operation of the other ionization source or the flow of analyte
ions toward the entrance of the mass spectrometer.
[0016] In multimode sources that include both an ESI source and an
APCI source (ESI/APCI), it is important that the downstream flow of
ions generated by the ESI source not substantially interfere with
either the corona discharge produced by the APCI corona needle or
the ions generated by the corona discharge. Such interference can
reduce the ion-generation efficiency of the APCI source and can
also reduce the number of APCI-generated ions that reach the
entrance of the mass spectrometer. In addition, the voltage levels
maintained at various portions of the multimode ion source
apparatus used to guide ions downstream and toward the entrance of
the mass spectrometer can influence the electric field at the
corona needle and thereby cause the corona discharge current to
vary, resulting in inconsistent operation of the APCI source.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 shows a general block diagram of a mass
spectrometer.
[0018] FIG. 2 shows an enlarged cross-sectional view of a first
embodiment of the invention.
[0019] FIG. 3 shows an enlarged cross-sectional view of a second
embodiment of the invention.
[0020] FIG. 4 shows an enlarged cross-sectional view of a third
embodiment of the invention.
[0021] FIG. 5 shows an enlarged cross-sectional view of a fourth
embodiment of the invention.
[0022] FIG. 6 shows an enlarged cross-section view of a fifth
embodiment of the invention.
[0023] FIG. 7 shows an enlarged cross-section view of a sixth
embodiment of the invention.
[0024] FIGS. 8A and 8B shows examples of infrared emitter lamps
that may be used in the context of the present invention.
[0025] FIG. 9 shows an enlarged cross-section view of a seventh
embodiment of the invention.
[0026] FIG. 10 shows an enlarged cross-section view of an eighth
embodiment of the invention.
[0027] FIG. 11A shows an example spectrum taken using an ESI/APCI
multimode source with only the ESI source being operated.
[0028] FIG. 11B shows an example spectrum taken using an ESI/APCI
multimode source with only the APCI source being operated.
[0029] FIG. 11C shows an example spectrum taken using an ESI/APCI
multimode source with both the ESI and APCI sources being
operated.
DETAILED DESCRIPTION
[0030] Before describing the invention in detail, it must be noted
that, as used in this specification and the appended claims, the
singular forms "a," "an," and "the" include plural referents unless
the context clearly dictates otherwise. Thus, for example,
reference to "a conduit" includes more than one "conduit".
Reference to an "electrospray ionization source" or an "atmospheric
pressure ionization source" includes more than one "electrospray
ionization source" or "atmospheric pressure ionization source". In
describing and claiming the present invention, the following
terminology will be used in accordance with the definitions set out
below.
[0031] The term "adjacent" means near, next to or adjoining.
Something adjacent may also be in contact with another component,
surround (i.e. be concentric with) the other component, be spaced
from the other component or contain a portion of the other
component. For instance, a "drying device" that is adjacent to a
nebulizer may be spaced next to the nebulizer, may contact the
nebulizer, may surround or be surrounded by the nebulizer or a
portion of the nebulizer, may contain the nebulizer or be contained
by the nebulizer, may adjoin the nebulizer or may be near the
nebulizer.
[0032] The term "conduit" refers to any sleeve, capillary,
transport device, dispenser, nozzle, hose, pipe, plate, pipette,
port, orifice, orifice in a wall, connector, tube, coupling,
container, housing, structure or apparatus that may be used to
receive or transport ions or gas.
[0033] The term "corona needle" refers to any conduit, needle,
object, or device that may be used to create a corona
discharge.
[0034] The term "molecular longitudinal axis" means the theoretical
axis or line that can be drawn through the region having the
greatest concentration of ions in the direction of the spray. The
above term has been adopted because of the relationship of the
molecular longitudinal axis to the axis of the conduit. In certain
cases a longitudinal axis of an ion source or electrospray
nebulizer may be offset from the longitudinal axis of the conduit
(the theoretical axes are orthogonal but not aligned in 3
dimensional space). The use of the term "molecular longitudinal
axis" has been adopted to include those embodiments within the
broad scope of the invention. To be orthogonal means to be aligned
perpendicular to or at approximately a 90 degree angle. For
instance, the "molecular longitudinal axis" may be orthogonal to
the axis of a conduit. The term substantially orthogonal means 90
degrees.+-.20 degrees. The invention, however, is not limited to
those relationships and may comprise a variety of acute and obtuse
angles defined between the "molecular longitudinal axis" and
longitudinal axis of the conduit.
[0035] The term "nebulizer" refers to any device known in the art
that produces small droplets or an aerosol from a liquid.
[0036] The term "first electrode" refers to an electrode of any
design or shape that may be employed adjacent to a nebulizer or
electrospray ionization source for directing or limiting the plume
or spray produced from an ESI source, or for increasing the field
around the nebulizer to aid charged droplet formation.
[0037] The term "second electrode" refers to an electrode of any
design or shape that may be employed to direct ions from a first
electrode toward a conduit.
[0038] The term "drying device" refers to any heater, nozzle, hose,
conduit, ion guide, concentric structure, infrared (IR) lamp,
u-wave lamp, heated surface, turbo spray device, or heated gas
conduit that may dry or partially dry an ionized vapor. Drying the
ionized vapor is important in maintaining or improving the
sensitivity of the instrument.
[0039] The term "ion source" or "source" refers to any source that
produces analyte ions.
[0040] The term "ionization region" refers to an area between any
ionization source and the conduit.
[0041] The term "electrospray ionization source" refers to a
nebulizer and associated parts for producing electrospray ions. The
nebulizer may or may not be at ground potential. The term should
also be broadly construed to comprise an apparatus or device such
as a tube with an electrode that can discharge charged particles
that are similar or identical to those ions produced using
electrospray ionization techniques well known in the art.
[0042] The term "atmospheric pressure ionization source" refers to
the common term known in the art for producing ions. The term has
further reference to ion sources that produce ions at ambient
pressure. Some typical ionization sources may include, but not be
limited to electrospray, APPI and APCI ion sources.
[0043] The term "detector" refers to any device, apparatus,
machine, component, or system that can detect an ion. Detectors may
or may not include hardware and software. In a mass spectrometer
the common detector includes and/or is coupled to a mass
analyzer.
[0044] The term "sequential" or "sequential alignment" refers to
the use of ion sources in a consecutive arrangement. Ion sources
follow one after the other. This may or may not be in a linear
arrangement.
[0045] The invention is described with reference to the figures.
The figures are not to scale, and in particular, certain dimensions
may be exaggerated for clarity of presentation.
[0046] FIG. 1 shows a general block diagram of a mass spectrometer.
The block diagram is not to scale and is drawn in a general format
because the present invention may be used with a variety of
different types of mass spectrometers. A mass spectrometer 1 of the
present invention comprises a multimode ion source 2, a transport
system 6 and a detector 11. The invention in its broadest sense
provides an increased ionization range of a single API ion source
and incorporates multiple ion formation mechanisms into a single
source. In one embodiment this is accomplished by combining ESI
functionality with one or more APCI and/or APPI functionalities.
Analytes not ionized by the first ion source or functionality
should be ionized by the second ion source or functionality.
[0047] Referring to FIGS. 1 and 2, the multimode ion source 2
comprises a first ion source 3 and a second ion source 4 downstream
from the first ion source 3. The first ion source 3 may be
separated spatially or integrated with the second ion source 4. The
first ion source 3 may also be in sequential alignment with the
second ion source 4. Sequential alignment, however, is not
required. The term "sequential" or "sequential alignment" refers to
the use of ion sources in a consecutive arrangement. Ion sources
follow one after the other. This may or may not be in a linear
arrangement. When the first ion source 3 is in sequential alignment
with second ion source 4, the ions must pass from the first ion
source 3 to the second ion source 4. The second ion source 4 may
comprise all or a portion of multimode ion source 2, all or a
portion of transport system 6 or all or a portion of both.
[0048] The first ion source 3 may comprise an atmospheric pressure
ion source and the second ion source 4 may also comprise one or
more atmospheric pressure ion sources. It is important to the
invention that the first ion source 3 be an electrospray ion source
or similar type device in order to provide charged droplets and
ions in an aerosol form. In addition, the electrospray technique
has the advantage of providing multiply charged species that can be
later detected and deconvoluted to characterize large molecules
such as proteins. The first ion source 3 may be located in a number
of positions, orientations or locations within the multimode ion
source 2. The figures show the first ion source 3 in an orthogonal
arrangement to a conduit 37 (shown as a capillary). To be
orthogonal means that the first ion source 3 has a "molecular
longitudinal axis" 7 that is perpendicular to the conduit
longitudinal axis 9 of the conduit 37 (See FIG. 2 for a
clarification). The term "molecular longitudinal axis" means the
theoretical axis or line that can be drawn through the region
having the greatest concentration of ions in the direction of the
spray. The above term has been adopted because of the relationship
of the "molecular longitudinal axis" to the axis of the conduit. In
certain cases a longitudinal axis of an ion source or electrospray
nebulizer may be offset from the longitudinal axis of the conduit
(the theoretical axes are orthogonal but not aligned in three
dimensional space). The use of the term "molecular longitudinal
axis" has been adopted to include those offset embodiments within
the broad scope of the invention. The term is also defined to
include situations (two dimensional space) where the longitudinal
axis of the ion source and/or nebulizer is substantially orthogonal
to the conduit longitudinal axis 9 (as shown in the figures). In
addition, although the figures show the invention in a
substantially orthogonal arrangement (molecular longitudinal axis
is essentially orthogonal to longitudinal axis of the conduit),
this is not required. A variety of angles (obtuse and acute) may be
defined between the molecular longitudinal axis and the
longitudinal axis of the conduit.
[0049] FIG. 2 shows a cross-sectional view of a first embodiment of
the invention. The figure shows additional details of the multimode
ion source 2. Multimode ion source 2 comprises a first ion source
3, a second ion source 4 and conduit 37 all enclosed in a single
source housing 10. The figure shows the first ion source 3 is
closely coupled and integrated with the second ion source 4 in the
source housing 10. Although the source housing 10 is shown in the
figures, it is not a required element of the invention. It is
anticipated that the ion sources may be placed in separate housings
or even be used in an arrangement where the ion sources are not
used with the source housing 10 at all. It should be mentioned that
although the source is normally operated at atmospheric pressure
(around 760 Torr) it can be maintained alternatively at pressures
from about 20 to about 2000 Torr. The source housing 10 has an
exhaust port 12 for removal of gases.
[0050] The first ion source 3 (shown as an electrospray ion source
in FIG. 2) comprises a nebulizer 8 and drying device 23. Each of
the components of the nebulizer 8 may be separate or integrated
with the source housing 10 (as shown in FIGS. 2-5). In the case
when the nebulizer 8 is integrated with the source housing 10, a
nebulizer coupling 40 may be employed for attaching nebulizer 8 to
the source housing 10.
[0051] The nebulizer 8 comprises a nebulizer conduit 19, nebulizer
cap 17 having a nebulizer inlet 42 and a nebulizer tip 20. The
nebulizer conduit 19 has a longitudinal bore 28 that runs from the
nebulizer cap 17 to the nebulizer tip 20 (figure shows the conduit
in a split design in which the nebulizer conduit 19 is separated
into two pieces with bores aligned). The longitudinal bore 28 is
designed for transporting sample 21 to the nebulizer tip 20 for the
formation of the charged aerosol that is discharged into an
ionization region 15. The nebulizer 8 has an orifice 24 for
formation of the charged aerosol that is discharged to the
ionization region 15. A drying device 23 provides a sweep gas to
the charged aerosol produced and discharged from nebulizer tip 20.
The sweep gas may be heated and applied directly or indirectly to
the ionization region 15. A sweep gas conduit 25 may be used to
provide the sweep gas directly to the ionization region 15. The
sweep gas conduit 25 may be attached or integrated with source
housing 10 (as shown in FIG. 2). When sweep gas conduit 25 is
attached to the source housing 10, a separate source housing bore
29 may be employed to direct the sweep gas from the sweep gas
source 23 toward the sweep gas conduit 25. The sweep gas conduit 25
may comprise a portion of the nebulizer conduit 19 or may partially
or totally enclose the nebulizer conduit 19 in such a way as to
deliver the sweep gas to the aerosol as it is produced from the
nebulizer tip 20.
[0052] It should be noted that it is important to establish an
electric field at the nebulizer tip 20 to charge the ESI liquid.
The nebulizer tip 20 must be small enough to generate the high
field strength. The nebulizer tip 20 will typically be 100 to 300
microns in diameter. In the case that the second ion source 4 is an
APCI ion source, the voltage at the corona needle 14 will be
between 500 to 6000 V with 4000 V being typical. This field is not
critical for APPI, because a photon source usually does not affect
the electric field at the nebulizer tip 20. If the second ion
source 4 of the multimode ion source 2 is an APCI source, the field
at the nebulizer needs to be isolated from the voltage applied to
the corona needle 14 in order not to interfere with the initial ESI
process. In the above mentioned embodiment (shown in FIG. 2) a
nebulizer at ground is employed. This design is safer for the user
and utilizes a lower current, lower cost power supply (power supply
not shown and described).
[0053] In one embodiment where the second ion source 4 is an APCI
ion source, an optional first electrode 30 and a second electrode
33 are employed adjacent to the first ion source 3 (See FIG. 2; For
further information regarding the electrodes described herein, See
Application No. 09/579,276, entitled "Apparatus for Delivering Ions
from a Grounded Electrospray Assembly to a Vacuum Chamber"). A
potential difference between the nebulizer tip 20 and first
electrode 30 creates the electric field that produces the charged
aerosol at the tip, while the potential difference between the
second electrode 33 and the conduit 37 creates the electric field
for directing or guiding the ions toward conduit 37. A corona
discharge is produced by a high electric field at the corona needle
14, the electric field being produced predominately by the
potential difference between corona needle 14 and conduit 37, with
some influence by the potential of second electrode 33. By way of
illustration and not limitation, a typical set of potentials on the
various electrodes could be: nebulizer tip 20 (ground); first
electrode 30 (-1 kV); second electrode 33 (ground); corona needle
14 (+3 kV); conduit 37 (-4 kV). These example potentials are for
the case of positive ions; for negative ions, the signs of the
potentials are reversed. The electric field between first electrode
30 and second electrode 33 is decelerating for positively charged
ions and droplets so the sweep gas is used to push them against the
field and ensure that they move through second electrode 33.
[0054] Since the electric fields are produced by potential
differences, the choice of absolute potentials on electrodes is
substantially arbitrary as long as appropriate potential
differences are maintained. As an example, a possible set of
potentials could be: nebulizer tip 20 (+4 kV); first electrode 30
(+3 kV); second electrode 33 (+4 kV); corona needle 14 (+7 kV);
conduit 37 (ground). Choices of potentials, though arbitrary, are
usually dictated by convenience and by practical aspects of
instrument design.
[0055] Use of APPI for second ion source 4 is a different situation
from use of APCI since it does not require electric fields to
assist in the ionization process. FIG. 4 shows a cross-sectional
view of an embodiment of the invention that employs APPI and that
is described in detail below. Although FIG. 5 shows the application
of the first electrode 30 and second electrode 33, optionally these
need not be employed with the APPI source.
[0056] The electric field between the nebulizer tip 20 and the
conduit 37 serves both to create the electrospray and to move the
ions to the conduit 37, as in a standard electrospray ion source. A
positive potential of, for example, one or more kV can be applied
to the nebulizer tip 20 with conduit 37 maintained near or at
ground potential, or a negative potential of, for example, one or
more kV can be applied to conduit 37 with nebulizer tip 20 held
near or at ground potential (polarities are reversed for negative
ions). In either case, the ultraviolet (UV) lamp 32 has very little
influence on the electric field if it is at sufficient distance
from the conduit 37 and the nebulizer tip 20. Alternatively, the
lamp can be masked by another electrode or casing at a suitable
potential of value between that of the conduit 37 and that of the
nebulizer tip 20.
[0057] The drying device 23 is positioned adjacent to the nebulizer
8 and is designed for drying the charged aerosol that is produced
by the first ion source 3. The drying device 23 for drying the
charged aerosol is selected from the group consisting of an
infrared (IR) lamp or emitter, a heated surface, a turbo spray
device, a microwave lamp and a heated gas conduit. It should be
noted that the drying of the ESI aerosol is a critical step. If the
aerosol does not under go sufficient drying to liberate the
nonionized analyte, the APCI or APPI process will not be effective.
The drying must be done in such a manner as to avoid losing the
ions created by electrospray. Ions can be lost by discharging to a
surface or by allowing the ions to drift out of the useful ion
sampling volume. The drying solution must deal with both issues. A
practical means to dry and confine a charged aerosol and ions is to
use hot inert gas. Electric fields are only marginally effective at
atmospheric pressure for ion control. An inert gas will not
dissipate the charge and it can be a source of heat. The gas can
also be delivered such that is has a force vector that can keep
ions and charged drops in a confined space. This can be
accomplished by the use of gas flowing parallel and concentric to
the aerosol or by flowing gas perpendicular to the aerosol. The
drying device 23 may provide a sweep gas to the aerosol produced
from nebulizer tip 20. In one embodiment, the drying device 23 may
comprise a gas source or other device to provide heated gas. Gas
sources are well known in the art and are described elsewhere. The
drying device 23 may be a separate component or may be integrated
with source housing 10. The drying device 23 may provide a number
of gases by means of sweep gas conduit 25. For instance, gases such
as nitrogen, argon, xenon, carbon dioxide, air, helium, etc. may be
used with the present invention. The gas need not be inert and
should be capable of carrying a sufficient amount of energy or
heat. Other gases well known in the art that contain these
characteristic properties may also be used with the present
invention. In other embodiments, the sweep gas and drying gas may
have different or separate points of introduction. For instance,
the sweep gas may be introduced by using the same conduits (as
shown in FIGS. 2 and 4) or different conduits (FIGS. 3 and 5) and
then a separate nebulizing gas may be added to the system further
downstream from the point of introduction of the sweep gas.
Alternative points of gas introduction (conduits, ports, etc.) may
provide for increased flexibility to maintain or alter
gas/components and temperatures. However, as noted above, a drying
gas may not be the sole or primary means used for drying the
aerosol. Embodiments employing an infrared emitter for drying the
aerosol are shown in FIGS. 6 and 7 discussed below.
[0058] The second ion source 4 may comprise an APCI or APPI ion
source. FIG. 2 shows the second ion source 4 when it is in the APCI
configuration. The second ion source 4 may then comprise, as an
example embodiment (but not a limitation), a corona needle 14,
corona needle holder 22, and coronal needle jacket 27. The corona
needle 14 may be disposed in the source housing 10 downstream from
the first ion source 3. The electric field due to a high potential
on the corona needle 14 causes a corona discharge that causes
further ionization, by APCI processes, of analyte in the vapor
stream flowing from the first ion source 3. For positive ions, a
positive corona is used, wherein the electric field is directed
from the corona needle to the surroundings. For negative ions, a
negative corona is used, with the electric field directed toward
the corona needle 14. The mixture of analyte ions, vapor and
aerosol flows from the first ion source 3 into the ionization
region 15, where it is subjected to further ionization by APCI or
APPI processes. The drying or sweep gas described above comprises
ones means for transport of the mixture from the first ion source 3
to the ionization region 15.
[0059] FIG. 3 shows a similar embodiment to FIG. 2, but comprises a
design for various points of introduction of a sweep gas, a
nebulizing gas and a drying gas. The gases may be combined to dry
the charged aerosol. As described above, the nebulizing and sweep
gas may be introduced as discussed. However, in this design the
drying gas may be introduced in one or more drying gas sources 44
by means of the drying gas port(s) 45 and 46. The figure shows the
drying gas source 44 and drying gas port(s) 45 and 46, comprising
part of second electrode 33. This is not a requirement and these
components may be incorporated separately into or as part of the
source housing 10.
[0060] FIG. 4 shows a similar embodiment to FIG. 2, but comprises a
different second ion source 4. In addition, in this embodiment, the
optional first electrode 30 and second electrode 33 are not
employed. The second ion source 4 comprises an APPI ion source. An
ultraviolet lamp 32 is interposed between the first ion source 3
and the conduit 37. The ultraviolet lamp 32 may comprise any number
of lamps that are well known in the art that are capable of
ionizing molecules. A number of UV lamps and APPI sources are known
and employed in the art and may be employed with the present
invention. The second ion source 4 may be positioned in a number of
locations downstream from the first ion source 3 and the broad
scope of the invention should not be interpreted as being limited
or focused to the embodiments shown and discussed in the figures.
The other components and parts may be similar to those discussed in
the APCI embodiment above. For clarification please refer to the
description above.
[0061] The transport system 6 (shown generally in FIG. 1) may
comprise a conduit 37 or any number of capillaries, conduits or
devices for receiving and moving ions from one location or chamber
to another. FIGS. 2-5 show the transport system 6 in more detail
when it comprises a simple conduit 37. The conduit 37 is disposed
in the source housing 10 adjacent to the corona needle 14 or UV
lamp 32 and is designed for receiving ions from the electrospray
aerosol. The conduit 37 is located downstream from the ion source 3
and may comprise a variety of material and designs that are well
known in the art. The conduit 37 is designed to receive and collect
analyte ions produced from the ion source 3 and the ion source 4
that are discharged into the ionization region 15 (not shown in
FIG. 1). The conduit 37 has an orifice 38 that receives the analyte
ions and transports them to another location. Other structures and
devices well known in the art may be used to support the conduit
37. The gas conduit 5 may provide a drying gas toward the ions in
the ionization region 15. The drying gas interacts with the analyte
ions in the ionization region 15 to remove solvent from the
solvated aerosol provided from the ion source 2 and/or ion source
3. The conduit 37 may comprise a variety of materials and devices
well known in the art. For instance, the conduit 37 may comprise a
sleeve, transport device, dispenser, capillary, nozzle, hose, pipe,
pipette, port, connector, tube, orifice, orifice in a wall,
coupling, container, housing, structure or apparatus. In certain
instances the conduit may simply comprise an orifice 38 for
receiving ions. In FIGS. 2-5 the conduit 37 is shown in a specific
embodiment in which a capillary is disposed in the gas conduit 5
and is a separate component of the invention. The term "conduit"
should be construed broadly and should not be interpreted to be
limited by the scope of the embodiments shown in the drawings. The
term "conduit" refers to any sleeve, capillary, transport device,
dispenser, nozzle, hose, pipe, plate, pipette, port, connector,
tube, orifice, coupling, container, housing, structure or apparatus
that may be used to receive ions.
[0062] The detector 11 is located downstream from the second ion
source 4 (detector 11 is only shown in FIG. 1). The detector 11 may
comprise a mass analyzer or other similar device well known in the
art for detecting the enhanced analyte ions that were collected and
transported by the transport system 6. The detector 11 may also
comprise any computer hardware and software that are well known in
the art and which may help in detecting analyte ions.
[0063] FIG. 5 shows a similar embodiment to FIG. 4, but further
comprises the first electrode 30 and the second electrode 33. In
addition, this embodiment of the invention includes the separation
of the sweep gas, nebulizing gas and drying gases. A separate
drying gas source 44 is employed as described above in FIG. 3 to
provide drying gas through drying gas ports 45 and 46.
[0064] Having described the invention and components in some
detail, a description of exemplary operation of the above-described
embodiments is in order. A method of producing ions using a
multimode ionization source 2 comprises producing a charged aerosol
by a first atmospheric pressure ionization source such as an
electrospray ionization source; drying the charged aerosol produced
by the first atmospheric pressure ionization source; ionizing the
charged aerosol using a second atmospheric pressure ionization
source; and detecting the ions produced from the multimode
ionization source. Referring to FIG. 2 as an exemplary embodiment,
the sample 21 is provided to the first ion source 3 by means of the
nebulizer inlet 42 that leads to the longitudinal bore 28. The
sample 21 may comprise any number of materials that are well known
in the art and which have been used with mass spectrometers. The
sample 21 may be any sample that is capable of ionization by an
atmospheric pressure ionization source (i.e. ESI, APPI, or APPI ion
sources). Other sources may be used that are not disclosed here,
but are known in the art. The nebulizer conduit 19 has a
longitudinal bore 28 that is used to carry the sample 21 toward the
nebulizer tip 20. The drying device 23 shown in FIG. 2, which
employs a flow of drying gas, may also introduce a sweep gas into
the ionized sample through the sweep gas conduit 25. The sweep gas
conduit 25 surrounds or encloses the nebulizer conduit 19 and
ejects the sweep gas to nebulizer tip 20. The aerosol that is
ejected from the nebulizer tip 20 is then subject to an electric
field produced by the first electrode 30 and the second electrode
33. The second electrode 33 provides an electric field that directs
the charged aerosol toward the conduit 37. However, before the
charged aerosol reaches the conduit 37 it is first subjected to the
second ion source 4. The second ion source 4 shown in FIG. 2 is an
APCI ion source. The invention should not be interpreted as being
limited to the simultaneous application of the first ion source 3
and the second ion source 4. Although, this is an important feature
of the invention. It is within the scope of the invention that the
first ion source 3 can also be turned "on" or "off" as can the
second ion source 4. In other words, the invention is designed in
such a way that the sole ESI ion source may be used with or without
either or both of the APCI and APPI ion source. The APCI or APPI
ion sources may also be used with or without the ESI ion
source.
[0065] FIG. 4 shows the second ion source 4 as an APPI ion source.
It is within the scope of the invention that either, both or a
plurality of ion sources are employed after the first ion source 3
is used to ionize molecules. In other words, the second ion source
may comprise one, more than one, two, more than two or many ion
sources that are known in the art and which ionize the portion of
molecules that are not already charged or multiply charge by the
first ion source 3. There are a number of important steps to make
the multimode ionizer operate. For instance, the effluent must exit
the nebulizer in a high electric field such that the field strength
at the nebulizer tip is approximately 10.sup.8 V/cm or greater.
This allows for the charging of the liquid molecules. The liquid is
then converted by the nebulizer in the presence of the electric
field to a charged aerosol. The charged aerosol may comprise
molecules that are charged and uncharged. Molecules that are not
charged using the ESI technique may potentially be charged by the
APCI or APPI ion source. The spray needle may use nebulization
assistance (such as pneumatic) to permit operation at high liquid
flow rates. As mentioned above the charged aerosol is then dried.
The combination of aerosol, ions and vapor is then exposed to
either a corona discharge or vacuum ultraviolet radiation. This
results in the second ion formation mechanism. Lastly, it is
important to maintain a voltage gradient in the source such that
the ions from both the ESI process and the second ion source are
directed into the conduit 37. The ions will then travel through the
transport system 6 to the detector 11 (transport system 6 is not
shown generally in the FIGS. 2-5).
[0066] FIG. 6 shows a similar embodiment to FIG. 2, in which the
drying device is implemented as an infrared emitter. As shown, an
inner chamber 50 has an opening 52 positioned adjacent to the
nebulizer tip 20 for receiving the charged aerosol from the ESI
source. The inner chamber extends longitudinally in the direction
of the molecular axis of the aerosol for some distance, and thereby
encloses the aerosol as it flows downstream.
[0067] The inner chamber 50 comprises an enclosure for an infrared
emitter 55 and may be of any convenient shape, size and material
suitable for sufficiently drying the aerosol it receives and
confining the heat generated by the infrared emitter 55 within its
enclosed space. Suitable materials may include stainless steel,
molybdenum, titanium, silicon carbide or other high-temperature
metals.
[0068] The inner chamber 50 includes an opening 56 for providing
exposure of the aerosol to the second atmospheric ionization
source. In FIG. 6, which shows an ESI/APCI multimode source, the
opening 56 allows the corona needle 14 to extend inside the inner
chamber 50. The opening 56 is dimensioned to allow sufficient
clearance for the corona needle, but is small enough to prevent an
appreciable amount of gases or heat from escaping. By having the
corona needle extend through the opening 56, the secondary
ionization of the analyte takes place within the inner chamber.
[0069] The inner chamber 50 also includes an exit 58 leading to the
exhaust port 12 and an interface 59 with the conduit 37. The
interface 59 to the conduit opening may be an orifice, or the inner
chamber may be sealingly coupled to the conduit 37 as shown. As the
aerosol is heated and the analyte ions are desolvated from solvent
molecules, the ions are attracted toward the conduit 37 via
electrical fields while the solvent molecules are urged by the
sweep of the aerosol toward the exhaust port 12. In the illustrated
embodiment, the optional first electrode 30 and second electrode 33
are not shown, but they may be included and positioned in an area
above the infrared emitter to aid in guiding the analyte ions
through the inner chamber toward the conduit. In addition, the
inner chamber may be grounded, or it may be maintained at a
positive or negative voltage for electric field shaping purposes
depending upon the polarity of the analyte ions.
[0070] The infrared emitter 55 is coupled to the inner chamber 50
and may comprise one or more infrared lamps that generate infrared
radiation when electrically excited. The infrared lamps may be of
various configurations and may also be positioned within the inner
chamber 50 in various ways to maximize the amount of heat applied
to the aerosol. For example, the infrared emitter may be configured
using "flat" lamps placed on opposite sides or ends of the inner
chamber and extending longitudinally along its length to achieve an
even distribution of radiation through the longitudinal length of
the chamber (while FIG. 6 illustrates a single coil, this coil may
be conceived of as one of a pair of lamps, the one illustrated
being situated at the "back" of the inner chamber recessed into the
page, and the other, not being illustrated, being in front of the
page). As an example of a lamp that can be used in this context,
FIG. 8A shows a shortwave flat lamp produced by Heraeus Noblelight
GmbH which is displayed on the Heraeus website at
http://www.noblelight.net. Alternatively, the infrared emitter may
be configured concentrically to surround a portion of the aerosol
as it flows through the inner chamber to promote radially symmetric
irradiation of the aerosol. FIG. 8B shows an example infrared lamp
which is coiled around a central tubular region and can be used in
a concentric configuration. An example of this configuration may
also be found displayed on the Heraeus Noblelight website.
[0071] It is useful for the infrared emitter 55 to emit peak
radiation intensity in a wavelength range that matches the
absoprtion band of the solvent used in the aerosol. For many
solvents, this absorption band lies between 2 and 6 microns. To
emit infrared radiation at such wavelengths, the lamps may be
operated at temperatures at or near 900 degrees Celsius. For
example, the radiation absorption band of water (approx. 2.6 to 3.9
microns) has a peak in the range of 2.7 microns, so that when water
is the solvent, it is advantageous to irradiate at or near that
wavelength to maximize heating efficiency. Other solvents, such as
alcohols and other organic solvents, may have absorption peaks at
longer wavelengths, and thus it is more efficient, when using such
solvents, to tune the peak infrared emission to longer wavelengths.
It is to be understood, however, that a portion of the radiation
emitted by the infrared emitter normally lies outside of this
"peak" band and encompasses both shorter and longer
wavelengths.
[0072] The intensity of the infrared emission from the lamps is
also controlled in a closed-loop manner to maintain the temperature
within the inner chamber in a suitable range for desolvating the
solvent molecules from the analyte ions. When the solvent is water,
the temperature within the inner chamber is typically maintained in
a range of about 120 to 160 degrees Celsius.
[0073] The inner surface of the inner chamber, which is exposed to
radiation emitted by the lamps, may be reflective with respect to
infrared radiation, by forming the inner chamber from a reflective
material, such as polished stainless steel, or by providing a
reflective coating on the inner surface. The reflective surface
improves heating efficiency since radiation that would otherwise be
absorbed by the surface of the inner chamber is reflected back
within the chamber, where such radiation may contribute to heating
and drying of the aerosol.
[0074] FIG. 7 shows a similar embodiment to FIG. 6, where the
second ion source 4 is an APPI ion source rather than an APCI
source. As shown, an ultraviolet lamp 32 is interposed between the
first ion source 3 and the conduit 37 and positioned adjacent to
the inner chamber 50. A UV-transparent window 57 is embedded within
a portion of the inner chamber wall facing the ultraviolet lamp 32
to provide for the exposure of the aerosol within the inner chamber
to the ultraviolet radiation emitted by the ultraviolet lamp 32.
The transparent window 57 may also be a screen, or orifice or any
other means for providing a sufficient dose of ultraviolet
radiation to the aerosol within the inner chamber. The ultraviolet
radiation further ionizes the molecules within the aerosol, and
importantly, may further ionize analyte species insufficiently
ionized by the ESI source.
[0075] FIG. 9 shows an ESI/APCI multimode source according to the
present invention in which the corona needle of the APCI source is
substantially enclosed by a corona needle shield device 65
(hereinafter the "shield"). The term "shield" should be construed
broadly however and should not be interpreted to be limited by the
scope of the embodiments shown in the drawings, described as
follows.
[0076] In the embodiment depicted, the corona needle 14 is oriented
orthogonally with respect to the molecular axis of the aerosol and
opposite from the conduit orifice 38, however, as noted above, this
orientation may be other than orthogonal. As shown in
cross-section, the shield 65 forms a cylinder that extends into the
ionization region for the about the length of the needle 14, and
has an end surface 67 with an orifice 68. The corona needle tip 16
terminates just inside the shield 65 before the orifice 68. The
diameter of the orifice 67 is dimensioned so that the electric
field at the corona tip 16 is considerably more strongly influenced
by the difference in voltage between the corona needle 14 and the
shield 65 than by the voltage difference between the corona needle
and the conduit 37, allowing the corona needle to be isolated from
the external electric fields. This has the benefit that corona
discharge current is relatively independent of the voltage applied
at the conduit 37. Moreover, the shield 65 physically isolates the
corona needle from the "wind" caused by the downstream flow or of
the ionized aerosol from the ESI source, which might otherwise
cause instability in the corona discharge, producing inconsistent
results.
[0077] To generate the electric fields required to produce a corona
discharge at typical voltage differences employed (e.g.,
approximately 3000 to 4000 V between the corona needle and the
shield), the diameter of the orifice 68 of the shield may be about
5 millimeters so that there is a 2.5 millimeter radial gap between
the tip and the end surface 67. The shield 65 can be operated at
ground or floated as needed to maintain a stable corona discharge.
However, these design parameters may be adjusted in accordance with
voltages applied, the ambient gas employed, and other factors as
would be readily understood by those of skill in the art.
[0078] It is also noted that while a drying device is not shown in
FIG. 9, any of the drying devices noted above including the
infrared emitter may be used in conjunction with the depicted
embodiment.
[0079] FIG. 10 shows an example of an ESI/APCI multimode source
according to the present invention in which an auxiliary electrode
70 is positioned adjacent to the APCI source corona needle 14 to
assist in guiding ions toward the conduit orifice 38 leading to the
mass analyzer (not shown). When the APCI source is used
simultaneously with the ESI source, the voltage on the corona
needle 14 may be high enough (in positive ion mode) to cause
positive ions flowing downstream to be repelled away from the
conduit orifice 38. The auxiliary electrode 70 is maintained at a
voltage of opposite polarity from and similar magnitude as the
corona needle. The voltage applied to the auxiliary electrode may
also be offset with respect to the conduit so that ions are guided
from the auxiliary toward the conduit orifice. As shown in the
exemplary illustration, the auxiliary electrode may be configured
as an extension of the conduit 37 and may be curved so that its end
is adjacent to the corona needle tip as shown. By positioning the
end of the auxiliary electrode adjacent to the corona needle, the
electric field lines become pinched in this region with the result
that the electric field strength and forces on the ions in this
region become very intense. Positive ions in the region of the
corona needle are thereby influenced strongly enough by this field
that the repulsion is overcome, and they are guided by the electric
field toward the conduit orifice.
EXAMPLES
[0080] FIG. 11A shows an example spectrum of an analyte sample
containing crystal violet and vitamin D3 obtained using a ESI/APCI
multimode source when only the ESI source is operated. As can be
discerned, only ions associated with crystal violet (372.2 and
358.2) are observed. In FIG. 11B, which shows an example spectrum
obtained from the same sample when only the APCI source is
operated, only the vitamin D3 related ions (397.3 and 379.3) are
observed. FIG. 11C shows an example spectrum obtained from the same
sample when both the ESI source and the APCI source are operated
simultaneously.
[0081] In this case both crystal violet ions (372.2, 358.2) and
vitamin D3 ions (397.3, 379.3) are observed, demonstrating the
effectiveness of using simultaneous operation of the two different
ionization modes in ionizing different chemical species.
[0082] It is to be understood that while the invention has been
described in conjunction with the specific embodiments thereof,
that the foregoing description as well as the examples that follow
are intended to illustrate and not limit the scope of the
invention. Other aspects, advantages and modifications within the
scope of the invention will be apparent to those skilled in the art
to which the invention pertains.
[0083] All patents, patent applications, and publications infra and
supra mentioned herein are hereby incorporated by reference in
their entireties.
* * * * *
References